The present invention is directed to an inspection system for ultrasonically inspecting a material such as metal, and, more particularly, the present invention is directed to a boresonic inspection system which performs shear mode inspection of near bore material in turbine and generator rotors by passing ultrasonic search units through an axial rotor bore.
For many years, there has been increasing interest in, and a growing demand for, equipment and methods which can be used to inspect power generation turbine and generator rotors for possible material discontinuities or degradation which could lead to premature, and possibly catastrophic, failure of these components and which allow rotor life extension where appropriate. The consequences of a sudden, catastrophic failure of such a component would be severe, certainly in financial terms and possibly in terms of human losses. The center portion of the steel forgings from which these rotors are made, by the very nature of the manufacturing process, is perhaps the most suspect material in the rotor in terms of naturally occurring discontinuities and other material disorders. This is, in fact, one reason that a central bore hole is machined through most rotors in an attempt to remove this suspect material. In addition, the operating conditions at and near the central bore holes in rotors can lead to service related disorders such as thermal creep, fatigue and thermal embrittlement, especially in the presence of inherent forging discontinuities. Thus, there is a great interest in rotor inspection capabilities.
Several nondestructive test methods have been developed for use in interrogating the bore and near bore regions of rotor forgings. When the forging is new and before the final machining has take place, it is still cylindrical or near cylindrical in shape and ultrasonic inspection from the outside has proven to be a valuable tool. However, because of the complex geometries which characterize the outer peripheries of completely machined forgings, ultrasonic inspection from the outside is impractical for inspecting rotors once they are machined. Other methods such as visual and magnetic particle examination have been used successfully to inspect the bore, but these methods are only sensitive to discontinuities which intersect or are very near to the bore and then only yield a two dimensional view of the material and any detected discontinuities.
Since the early to mid 1970's, ultrasonic inspection from the rotor bore itself has gained fairly wide acceptance as viable volumetric inspection method. In this method, which has become known as boresonic inspection, the ultrasonic transducers are transported through the central bore hole by some convenient method and the ultrasonic beams are directed from the bore surface into the rotor material. The ultrasonic wave can penetrate well into the rotor material, and by collecting, processing, and observing any reflections of the wave which occur within the forging, one can get some idea of the integrity of the material. Volumetric inspection is achieved by scanning the transducers around the circumference and along the length of the bore while directing the ultrasonic beam into the material so that the beam has been ultimately passed through all of the material of interest.
Early boresonic test systems and some still in use, such as that described in U.S. Pat. No. 3,960,006, are based on conventional, contact ultrasonic practices.
More recently, a new direction regarding bore ultrasonic inspection of rotors has begun to emerge. A test system, known as TREES (Turbine Rotor Examination and Evaluation System) has been developed under the direction of the Electric Power Research Institute (EPRI) for the American Electric Power Company. This test system is the first known rotor bore inspection system to provide inspection capability based upon immersion ultrasonic testing techniques. For the purpose of this writing, TREES is categorized as a fixed focus immersion system.
Fixed focus immersion systems provide certain features which overcome many of the shortcomings of the contact systems. The transducers operate in an immersion bath which eliminates many, if not all, of the contact problems. No transducer shoes are required as the water provides a path for the sound to travel from the transducer to the rotor. The transducer can be offset from the bore by an amount which provides for the near field effects to occur entirely in the water so that the beam is formed and well behaved at the bore surface and beyond. Generation of either angled compressional or shear waves in the rotor can be easily accomplished by simply tilting the transducer such that the beam strikes the bore surface at other than normal incidence.
In the prior art, manual, pneumatic and motor driven inspection systems, the control systems that move the scan head and provide position indications have been cumbersome and inaccurate due to resolver locations that require knowledge of mechanical slack in the system and positioning apparatus that does now allow for high resolution positioning. As a result, the location and size of discontinuities and flaws have been inaccurately located. Inaccurate flaw location, requires that remachining to remove flaws cover a larger area than is necessary, weakening the rotor at its highest stress area, near the bore. Inaccurate flaw location also hinders comparison of previous inspections with current inspections because it is difficult to determine whether a given flaw is a new flaw or an old flaw that has been inaccurately located due to alignment inaccuracies.
See U.S. Pat. No. 4,757,716 for additional discussion relating to the background of this invention.